This invention relates generally to transducers for inducing vibratory signals in an elastic medium such as the earth and, more particularly, an improved means for coupling the horizontally-directed force of a seismic transducer into the earth.
The closest prior art known to applicant is exemplified by U.S. Pat. No. 4,135,599, wherein the horizontal force of a vibrator is coupled into the earth by means of a pair of identical generally pyramidal cleats mounted on the underside of the vibrator support means. The main lateral load-bearing plates are trapezoidal, while the end plates are triangular. The lateral and end plates of these cleats all converge to form cleat edges aligned transversely to the direction of the applied horizontal vibrator force and along the direction of travel of the transporting vehicle.
This prior art patent suggests that the character of the earth surface to be engaged should govern the choice of overall shape, cleat edge length, and total area of the lateral load-bearing surfaces of the cleats. But since all the cleats are identical and interchangeable, the configuration which works well with one type of earth material may be inefficient with others.
For example, cleats constructed according to this prior art design may be sharp enough to perform well on surfaces ranging from bedded limestone to firm clay soils. However, on sandy soil, i.e. loose soil, the same cleats may work down rapidly during each horizontal sweep of the vibrator so that much or all of the weight of the vibrator is transferred to the vibrator support means itself at which point little horizontal earth motion can be generated by the vibrator.
The use of a blunter prior art cleat construction in the illustrative situation above would reduce this rate of penetration in sandy soils and hence, the likelihood of "bottoming out". But this, in turn, would reduce the rate of penetration and efficiency in rock (i.e. harder materials), where more slippage would result and more unwanted vertical force would be generated, and thus create undesirable compressional waves and undesirable higher harmonics in the shear waves.
Another problem inherent in prior art cleat designs of the type described is that the front and back or end cleat surfaces (i.e., those parallel to the direction of vibrator motion) are essentially inactive, in that they do not aid in providing the desired horizontal shear wave coupling to the earth. However, because they slope away from the vertical when the cleats are imbedded, these sloping surfaces take up part of the total vertical force available to hold the cleats firmly against the earth. As a result, less force is available to compress unconsolidated earth materials adjacent to the surfaces of the active plates. Compression of the earth adjacent to the active surfaces is desirable because compression increases the shear strength of unconsolidated earth materials. Any decrease in the compression adjacent active surfaces decreases the shear strength of the earth in this vicinity, so the horizontal stress which can be transmitted downward through the earth is correspondingly reduced.
It is, therefore, a general objective of this invention to provide an improved means of coupling the horizontally directed force of a transducer into an elastic medium such as the earth.
It is another object of this invention to provide means for coupling the horizontally-directed force of a transducer into the earth which are adapted for use in soils varying widely in competency.
It is yet another object of this invention to provide means for coupling of the horizontal shear wave energy of a transducer into the earth which is adapted to prevent premature "bottoming out" of the transducer.
It is a still further object of this invention to provide means for coupling the horizontal shear wave energy of a vibrator into the earth adpated to generate less unwanted compressional wave energy and less unwanted higher harmonics in the shear wave.
Other and further objects and advantages of the invention will become apparent upon consideration of the following detailed description and drawings.
In summary, in accordance with a preferred embodiment of the present invention, means are provided for coupling the energy of a horizontal vibrator into the earth comprising a plurality of downwardly convergent earth-engaging cleats dependently supported from said transducer. The cleats are in the form of inverted wedges each of whose rectangular, lateral load-bearing plates are convergently joined along a common edge to form a cleat edge aligned transversely to the direction of the applied horizontal force, and whose triangular opposed end plates are normal to the horizontal plane of such edges. The cleats may differ in construction as to their depth, sharpness, and edge length. Preferably, the sharper cleats are the deeper ones, so that they are adapted to penetrate hard surfaces more readily. Also, the lateral extent of the various cleats is preferably such that the total effective edge length of the sharper cleats is less than that of the blunter ones, thereby insuring a higher vertical loading on the sharper cleats.
In an alternate embodiment of this invention, the lateral or earth-coupling plates of the cleats are trapezoidal and the triangular end plates are sloped from the normal so as to provide a further means for varying the total effective edge length of the cleats.